1. Field of the Invention
The present invention relates to a light emitting device and driving circuit thereof and, more particularly, to a light emitting device and driving circuit thereof that can improve a power factor and optical efficiency while reducing total harmonic distortion and flickering.
2. Discussion of the Background
Light emitting diodes (LEDs) also exhibit common characteristics of diodes that are turned on upon application of a forward threshold voltage or more thereto. Further, two or more LEDs may be connected in inverse parallel with each other in order to increase a light emitting region upon application of an AC voltage source (hereinafter, the connected LEDs will be referred to as an “AC LED”). In this case, in a positive half-period of the AC voltage source, the AC LED is turned on by application of a forward threshold voltage or more to the LEDs connected to each other in the forward direction with respect to the positive half-period of the voltage, and in a negative half-period of the AC voltage source, the AC LED is turned on by application of a forward threshold voltage or more to the LEDs connected to each other in the forward direction with respect to the negative half-period of the voltage.
When applying the AC voltage source, each of the LEDs has a short operating region, which causes a problem of deterioration in optical efficiency of the AC LED by severe flickering or total harmonic distortion. Such problems may become severe when multiple AC LEDs are connected in series. The problems of the AC LED will be described hereinafter with reference to the drawings.
FIG. 1 is an equivalent circuit diagram of a conventional AC LED, and FIG. 2 is a graph depicting voltage-current characteristics of the AC LED shown in FIG. 1.
Referring to FIG. 1, a light emitting device 10, an AC voltage source Vac, and a resistor R11 are connected in series with one another. Here, LED 12 (D11, D12) and LED 14 (D13, D14) will be referred to as AC LEDs.
When a positive half-period of the AC voltage source Vac is applied to AC LED 12 and AC LED 14, LED D11 and LED D13 are operated. It should be understood that, since the LED D11 and LED D13 are connected in series, LED D11 and LED D13 are operated when the voltage is greater than the sum of forward threshold voltages of LED D11 and LED D13.
Similarly, when a negative half-period of the AC voltage source Vac is applied to AC LED 14 and AC LED 12, LED D14 and LED D12 are operated. In this case, LED D14 and LED D12 are operated when the voltage is greater than the sum of the forward threshold voltages of LED D14 and LED D12. Herein, operation of the LEDs will be construed as referring to light emission operation of the LEDs in the following description.
When AC LED 12 and AC LED 14 are operated in the positive or negative half-period of the AC voltage source Vac, a current is dependent on the resistor R11.
In FIG. 2, v1 is a voltage graph and i1 is a current graph. The x-axis indicates time and the y-axis indicates the intensity of current or voltage. This will be identically applied to all of the following voltage and current graphs.
As described in FIG. 1, in application of the AC voltage source Vac to the AC LEDs, a current is allowed to flow through the AC LEDs when the voltage is greater than the sum of the forward threshold voltages of the respective LEDs connected in a forward direction with respect to the AC voltage source Vac according to the positive or negative half-period of the AC voltage source Vac. Such characteristics are clearly shown by the voltage-current graphs of FIG. 2. It should be understood that, when the light emitting device comprises a single AC LED 12 or AC LED 14, it also exhibits similar voltage-current characteristics to the light emitting device described above. Furthermore, although two AC LEDs, LED 12 and LED 14, are shown in FIG. 1, a light emitting device comprising three or more AC LEDs also exhibits similar voltage-current characteristics to those of FIG. 2.
Such characteristics of AC LED 12 and AC LED 14 operated only by an AC voltage higher than or equal to the sum of the forward threshold voltages cause several problems. In other words, when the AC voltage source Vac applied to AC LED 12 and AC LED 14 is higher than or equal to the sum of the forward threshold voltages of the LEDs connected in the forward direction with respect to the voltage, a current flows through the AC LEDs suddenly, and a short operating region is provided to the AC LEDs for a single period of the AC voltage source applied thereto, thereby causing an increase in total harmonic distortion (THD), excessive flickering, and deterioration in optical efficiency.
Therefore, there is an urgent need for a light emitting device or driving circuit thereof that can solve problems caused by the operating characteristics of the AC LED upon application of an AC voltage source, such as power factor decrease, total harmonic distortion, and excessive flickering.